Field of the Invention
The present invention relates to an image pickup apparatus, a control method therefor, and a storage medium, and particularly to an image pickup apparatus that performs exposure at a timing at which a change in brightness caused by flicker is small.
Description of the Related Art
Recently, it is possible to shoot an image without any blur even with a high-speed shutter under a relatively dark environment such as an interior along with an increase in sensitivity of an image pickup device to be used in an image pickup apparatus such as a digital camera. On the other hand, a so-called flicker phenomenon in which illumination light cyclically fluctuates is generated due to influence of a commercial power frequency in a fluorescent light which is used as an indoor light source. Further, when the shooting is performed with the high-speed shutter under a flicker light source such as the fluorescent light, a variation in exposure and color temperature of an image is generated for each frame due to the influence of the flicker. Furthermore, there is a case in which unevenness in exposure and unevenness in color are generated in a single frame.
An image pickup apparatus, which detects a period and a phase of flicker and performs exposure in accordance with a timing at which a change in brightness is small, is proposed in order to deal with the above-described problems (see Japanese Laid-Open Patent Publication (kokai) No. 2007-60585).
Meanwhile, when time elapses after detecting the flicker, a relation between an optimal exposure timing, which is determined at the time of flicker detection, and an optimal exposure timing at the time of shooting deviates due to the influence of a fluctuation in the commercial power frequency in some cases. In general, it is said that a fluctuation in the commercial power frequency is about ±0.3 Hz (a fluctuation in a flicker frequency of a flicker light source is about ±0.6 Hz). Thus, there is a possibility that an exposure timing deviates as time elapses after a flicker detection even if the flicker detection is once performed to obtain an exposure timing at which the change in brightness caused by the flicker is small.
It is necessary to frequently perform the flicker detection with a cycle in which the deviation of exposure timing does not occur in order to deal with such a fluctuation in the commercial power frequency. However, a normal photometric frequency decreases when the flicker detection is frequently performed. Further, it is difficult to perform an exposure control in real time when the normal photometric frequency decreases. On the other hand, it is necessary to decrease detection frequency of a subject to prevent the normal photometric frequency from decreasing, for example, in order to perform the exposure control in real time.
In this manner, the frequency of the normal photometry or the subject detection decreases when the frequency of the flicker detection increases, which prevents a user from obtaining a desired image.